[TAQuest]

Originally Posted by 78w72

i plan to put a visible gauge on mine next time at the track to watch what the pressure does during the run. what is the minimum psi it needs to maintain on a run to confirm its not a fuel issue?

Depends on what the carb wants. They are all different. Have to be specific on which carb.

[Formulajones]

I didn't see any mention of a gauge on this setup Rock. Would be nice if you could monitor that while these issues appeared, and it may have even told you of a problem early on under normal driving conditions if you saw pressure gradually degrading over time.

First I'll say I really don't have issues with mechanical pumps. Even stock pumps. I daily drive them, drag race them etc...and have gone mid 12's at 108-110 mph with them in 4,000 lbs. cars using the stock fuel lines and stock pumps. I'm still doing that to this day as a matter of fact in both my firebird and our Z that are both daily driven throughout the year in the sizzling AZ heat, which is over 100 degrees right now. They are simply vented at the cap. However I use NOS caps because the reproduction crap out there doesn't vent properly. No vapor lock issues with either car. There are so many details that need to be right for this stuff to be reliable that I won't even get into it here. It's just too much to explain.

Second, I know guys with the Robbmc 550 pumps on their Pontiacs that are running deep into the 11's at 116+ mph. There are other ways to do the fuel system for sure, but there are plenty getting it done with this setup. Nice thing about the Robb pumps is they are rebuildable and he offers the kits separately for about $25.

However with all that said, If I'm building a car that doesn't have to conform to rules or originality and stock appearances aren't a concern, then I go right for the in tank electric fuel setups, 1/2" lines etc... and even this stuff can be concealed or made to appear stockish. Setups like this are pretty much OEM reliable now and in my opinion the only way to go on any classic car that wants to make some power and have a rock solid fuel system to support it. It'll cost about $1,000 to set it all up properly but it's something you'll never have to concern yourself with again.

[RocktimusPryme]

Originally Posted by 78w72

yeah sounds like something may have failed if these symptoms never showed up before.

i considered the cap before too, bought a new vented cap & verified it is indeed venting by sucking through it. another trick is to drill an extra hole in the cap for even more venting, but if the tank is full you may get some sloshing of fuel on launch.

what HP is your engine? consider upgrading the pump for ~$100 if you can afford it & your engine needs it. the "550" rating is for an engine sitting still on a run stand with a short direct fuel line, once you put it in a car all that goes out the window & maybe you need a bigger pump?

best of luck!

It made 580 on the engine dyno, in the car I figure its more like 530 with fan and all that jazz. I am considering upgrading, but Im wondering how necessary the upgraded mechanical pump is if I add an electric auxiliary pusher. Does that render it pointless? IDK. Ill talk to Robb about it.


Im also curious why his regulators say that the return style are for FI only, and the dead head is for carbs.

[78w72]

Originally Posted by TAQuest

Depends on what the carb wants. They are all different. Have to be specific on which carb.

myself & rocktimus are running a q-jet as mentioned above...

[Formulajones]

Originally Posted by 78w72

i plan to put a visible gauge on mine next time at the track to watch what the pressure does during the run. what is the minimum psi it needs to maintain on a run to confirm its not a fuel issue?

From my experience, when pressures start dropping below ~4psi you are likely starting to struggle keeping the bowl or bowls full and you start to see performance issues, maybe not laying over completely, but mph does suffer. Q-jets need a more consistent supply.

Mike on the forum here was running a 550 RobbMC pump on his 455 bird and running mid 11's at 116 mph, and his fuel pressure was dropping to 3psi if I remember correctly, but was making clean passes, likely thanks to the dominator carb and more fuel capacity in reserve. But he knew he had fuel delivery issues creeping in and tried several different configurations trying to solve it. I think the car had more in it, and he did too.
Really helps to have a gauge mounted front and center to monitor these things. Every hot rod should have one permanently mounted in my opinion.

[Formulajones]

Originally Posted by RocktimusPryme

Im also curious why his regulators say that the return style are for FI only, and the dead head is for carbs.

I don't follow that logic at all. The factory had thousands of return style fuel systems on carbs. My firebird is one, my chevelle, etc... Only our Z is dead headed from the factory.

Personally I prefer the return style systems. Keeping that fuel circulating during idle and low throttle situations when the needle and seat are virtually closed is a huge player in keeping that fuel cool and not sitting stagnant in the fuel lines attracting heat. Todays pump gas fuels are more susceptible to that than they ever were before.

[78w72]

Originally Posted by RocktimusPryme

It made 580 on the engine dyno, in the car I figure its more like 530 with fan and all that jazz. I am considering upgrading, but Im wondering how necessary the upgraded mechanical pump is if I add an electric auxiliary pusher. Does that render it pointless? IDK. Ill talk to Robb about it.


Im also curious why his regulators say that the return style are for FI only, and the dead head is for carbs.

yeah at 580hp you definitely need the bigger 1100 pump IMO. robb told me the 550 was too small for my ~525hp (on a tight dyno) once you have almost 20ft of fuel line with bends & then launch at the track, those pump ratings are worthless. im sure you know that, just saying at your power level the 550 is probably not enough, then add in the super small fuel bowl of the q-jet & its a struggle to keep it full.

if you commit to adding a pusher pump then you probably dont need to upgrade the mech pump, especially if you dont have any issues on the street & will rely on the pusher to fix the problems at the track. but for ~$100 it might be cheaper & easier to upgrade to a bigger pump, no downside & you're already sending him the pump.

on the return issue, the way i understand it is that robbs pumps/regulators dont have a true return line for carbs, its just a vapor return & uses a very small orifice & line, a true fuel return needs high flow & big lines.

[TAQuest]

Originally Posted by RocktimusPryme

It made 580 on the engine dyno, in the car I figure its more like 530 with fan and all that jazz. I am considering upgrading, but Im wondering how necessary the upgraded mechanical pump is if I add an electric auxiliary pusher. Does that render it pointless? IDK. Ill talk to Robb about it.


Im also curious why his regulators say that the return style are for FI only, and the dead head is for carbs.

You don't want two pumps.

You are still guessing.

Get a gauge and watch it when you drive.

[68lemans462]

Originally Posted by 78w72

i plan to put a visible gauge on mine next time at the track to watch what the pressure does during the run. what is the minimum psi it needs to maintain on a run to confirm its not a fuel issue?

6'ish psi is good with a q-jet

[tom s]

I have been running a Carter street pump with a switched elect in the back forever.I run 1/2in from the tank to the mech pump and 3/8 hard line to a pair of 750 AFBs.Car ran 112 in the 1/4 and never had a fuel issue.For street use I dont run the elect.Just use it to prime and if im going to run the car.FWIW,Tom

[78w72]

Originally Posted by Formulajones

From my experience, when pressures start dropping below ~4psi you are likely starting to struggle keeping the bowl or bowls full and you start to see performance issues, maybe not laying over completely, but mph does suffer. Q-jets need a more consistent supply.

Mike on the forum here was running a 550 RobbMC pump on his 455 bird and running mid 11's at 116 mph, and his fuel pressure was dropping to 3psi if I remember correctly, but was making clean passes, likely thanks to the dominator carb and more fuel capacity in reserve. But he knew he had fuel delivery issues creeping in and tried several different configurations trying to solve it. I think the car had more in it, and he did too.
Really helps to have a gauge mounted front and center to monitor these things. Every hot rod should have one permanently mounted in my opinion.

thanks. thats the psi info i was asking about. i remember reading it had to stay at 4-5psi on the run or starvation was a possibility. 3psi seems pretty low but the holley/dominator has a big advantage over a q-jet in bowl size.

i plan to rig up a visible gauge for some testing next time out. i know there is more in the car if i can get past the fuel issue. & there should be a LOT more in rocktimus car if he can figure out whats causing his problems.

[TAQuest]

Originally Posted by 78w72

thanks. thats the psi info i was asking about. i remember reading it had to stay at 4-5psi on the run or starvation was a possibility. 3psi seems pretty low but the holley/dominator has a big advantage over a q-jet in bowl size.

i plan to rig up a visible gauge for some testing next time out. i know there is more in the car if i can get past the fuel issue. & there should be a LOT more in rocktimus car if he can figure out whats causing his problems.

When it's right it doesn't fluctuate. Should stay at what the pump relief or regulator is set at.

[HWYSTR455]

Basic rule of thumb, if under demand, you drop below 5.5 or so, there is a delivery deficiency.

There's always some fluctuations while in use, due to the demand/no demand rollercoaster. You can compensate for that by starting at a higher psi than the minimum. Like 7-9 psi, or as high as the needle/seat can handle. That way, by the end of a 1/4 mile for example, you still have some reserve.

Optimally, your fuel psi should never drop below 6-7 psi, or whatever your predetermined psi is. Ever.

So when you are designing a system, what would be your goal(s)? So you have 'just' enough to make it to the end of the 1/4, or to have optimum under all conditions?

Well, if you never want to ever have an issue, or have to worry that you MIGHT have an issue, which would you choose? Just enough, or optimum?


.

[TAQuest]

Originally Posted by HWYSTR455

There's always some fluctuations while in use,




.

Nope.
I have the video to prove it. Not supposed to fluctuate. That's what the relief valve or regulator is for. You don't want any fluctuation.

[TAQuest]

If your pump is right and your system is right then you should always have more than enough and the regulator in the pump or inline takes it down to what you need.

[RocktimusPryme]

Originally Posted by Formulajones

I didn't see any mention of a gauge on this setup Rock. Would be nice if you could monitor that while these issues appeared, and it may have even told you of a problem early on under normal driving conditions if you saw pressure gradually degrading over time.

I don't have a pressure gauge but I do have a wideband. I did feel like it had been running a little leaner under normal driving lately. It was one of those things where you start questioning yourself "Did it used to be like that?" "Maybe its just the weather"

But again, it appeared to get leaner over the last couple weeks and worse on the way home from the track. Which is why I feel like a specific component is going bad.

Unfortunately its mounted low, so I couldn't see it realtime on the strip. yeah, yeah I know, but it is what it is. By the time I got my head around too it I had already let off the gas and the AFR was in the 12s or so again.

[RocktimusPryme]

Originally Posted by TAQuest

You don't want two pumps.

You are still guessing.

Get a gauge and watch it when you drive.

Well its showing signs of detonation now, so Im not going to drive it again without changing something. I don't want to hurt it. Other than that, I generally agree.

The more i think about it, I think Im going to take one step at a time. Get the pump inspected, probably just upgrade the damn thing and buy a regulator and gauge. Then if its still having issues Ill go from there.

I also might take my spare 750 holley off the rack and take it to the track with me just to see.

[mchell]

As usual, you will get a gazillion fuel delivery opinions....in the end you will need to do whatever will work on YOUR car...what works for others(or not work) may not work for you........that being said, here’s another opinion:

I never had luck running a mechanical pump without a pusher, and that included the Robb mc 1100.....finally went back to an electric and never looked back. I am still running a non sumped stock tank with 1/2 pickup and lines....it works, but I’m certain it’s not optimal by any stretch

Definitely get a gauge hooked up.

[HWYSTR455]

Originally Posted by TAQuest

Nope.
I have the video to prove it. Not supposed to fluctuate. That's what the relief valve or regulator is for. You don't want any fluctuation.

Regulators don't react immediately, some really good ones are 'fast', but it's still not 'instant'.

Gauges are the same way, they don't react instantly, and many are 'padded' or 'buffered', to prevent needle 'shake'.

You can post that vid, but that only tells me the gauge is not very good.

A transducer is the fastest reacting, try one of those, gather the data using a data logger and then look at the output charted. Then compare that to your gauge. Tell me what you see.

.

[ta man]

A gauge is a must. Without a sump in my car I need at least a half a tank of fuel at the track..what the gauge does is monitor that there is sufficient pressure. What you want ideally to see is your set pressure all the way down the track.
On a side note, I've never understood the "bigger bowl" theory for covering up weak fuel system..once the bowl level drops your carb isn't working properly and starving for fuel. After doing a good burnout and then a hard 11 or 12 second run..one will use quite a few "bowls" of fuel and if the fuel supply isn't up to snuff that float level just keeps dropping.